The reason you can bleed to death is that loss of blood means loss of haemoglobin in your blood. This prevents cells from getting oxygen to produce energy and so they starve to death in your body.

The flip side to this is that too much iron in the body can cause Fe to form iron solids in body tissues, which is not healthy either. Too much iron has been implicated in causing cardiovascular disease.

It is important to have a balanced level of Fe in your body.

Food is the usual source of iron in the body, although dietary supplements are available. These foods are high in iron:

Red meat

Liver

Dark leafy greens

Dried fruit

Egg yolks

Eating foods high in Vitamin C at the same time as eating iron-rich foods will help iron be absorbed into the body. (Most fruits are high in Vitamin C, particularly oranges, kiwifruit, and also red peppers).

Iron Reactions

However, Fe can be very dangerous in the body if it is released from haemoglobin. As already discussed in the post regarding malaria, free iron will react with hydrogen peroxide to create free radicals that attack DNA, killing cells and making you sick.

Fenton Reaction produces dangerous radical molecules in the body

The only place haeme (an iron centre with the immediate surrounding protein) from haemoglobin can be safely disposed of is in the liver, where a chemical pathway is available to recycle red blood cells. Haemoglobin is broken down into Bilirubin.

Bilirubin can then be broken down by certain wavelengths of light as well as enzymes in the body.

Neonatal hyperbilirubinaemia is a condition where newborn babies turn yellow because they cannot break down this yellow bilirubin.

This condition can be fixed by putting the baby under special blue lights. This light causes the bilirubin to break down.

Phototherapy used to treat a baby with jaundice.
Photo: Wikipedia Commons, Rjmunro

Iron is used for more than just oxygen transport in the body. Bacteria, yeast and fungi also need iron to live.

Interestingly, the first layer of protection against bacterial infection is at the entry points to the body: mouth, nose etc. Lactoferrin is an antibiotic produced in saliva and snot, which strips Fe from bacteria.

The idea is that if bacteria need Fe to reproduce and infect you, take their Fe off them and they will die.

However, bacteria have chemicals of their own. Siderophores, another chemical that binds strongly to iron helps bacteria to capture iron from their environments.

Siderophores are used to remove iron from transferrin iron transfer and storage proteins in the body. The siderophore is then absorbed by bacteria that breaks down the siderophore and stashes a source of iron for themselves.

Electron Micrograph of a colony of E. Coli bacteria.
Photo: Eric Erbe, US Department of Agriculture

Enterobactin (excreted by E. coli) is an extremely efficient siderophore. It is so strong, it can remove iron from glass.

Once the first-line defences have failed, the body’s next response to a bacterial invasion is the production of a molecule called siderocalin. Siderocalin binds enterobactin and so prevents bacteria getting any iron, killing them.

However, bacteria are developing resistance to siderocalin and so the battle for iron- and life- continues.

To find out further information, have a look at the Public Library of Science article ‘The Battle for Iron between Bacterial pathogens and their Vertebrate Hosts’, available at:

These blood cells transport oxygen around all parts of the body using the blood system.

Haemoglobin is a type of protein. It is made up of four different parts. Each of these parts has a haem ring with a Fe atom in its centre. This is chemically attached to the protein by four strong bonds.

Note: a ‘chemical bond’ means that the two atoms involved share electrons.

Iron is capable of forming 6 bonds. In haemoglobin it sits within a haem ring, binding to four amino acids. A molecule of oxygen will attach at one of the remaining empty sites.

Haemoglobin: A ribbon representation of the protein. The 4 green areas show the Fe haem centres where oxygen will bind. Diagram: Wikipedia Commons, first uploaded by Zephyris and adapted by Richard Wheeler

An intricate balancing system is required so that oxygen will bind to Fe in haemoglobin (to be transported) but when cells around the blood system need it, the oxygen will be released.

Basically, the strength of the Fe-O bond has to change based on how much oxygen is around. Scientists have tried to understand how this works by using a ‘spring-tension’ model.

The reason that inhaling CO (carbon monoxide) is fatal, is because CO will bind to haemoglobin molecules. This prevents the transport of oxygen to cells which in turn prevents cell respiration and effectively starves cells to death.

The Spring-Tension Model

This model says that a haemoglobin molecule changes its shape depending on the concentration of oxygen around it.

It’s sort of like a crab claw.

Diagram of the Spring Tension Model, used to understand haemoglobin and oxygen binding in high and low oxygen concentrations. Source: Lecture notes from University College Dublin course: ‘Metals in Biology’

In the lungs and other areas where there is lots of oxygen, each Fe centre in the haemoglobin forms a strong bond to an oxygen molecule. This can be compared to a crab closing its pincher around a piece of food.

The formation of a Fe-O bond (an oxygen molecule attaches to the haemoglobin molecule) causes the protein to change shape, becoming more compact. This signals the other three Fe centres in the protein to bind oxygen quickly.

This is like signalling the other pincher of the crab to close, getting a better grip on the crab food.

In areas where there is not much oxygen, the presence of carbon dioxide gas signals the haemoglobin to release oxygen. The Fe-O chemical bonds between the haemoglobin iron centre and the oxygen become much weaker.

Once one Fe-O bond has broken, the protein gets bigger and this triggers the other Fe centres again to release their oxygen.

In our crab analogy, the crab looks at what is in its pinchers and decides it would prefer to eat something else. The crab releases its pinchers, one at a time.